This invention relates generally to the field of superconductors, and more specifically, the physical properties of the structural and insulating material used with superconductors, and in particular, the coefficient of thermal expansion of the material.
Glass reinforced epoxy insulation structures are often used in superconducting magnets. Large magnets using Cable-in-Conduit-Conductor (CICC) require insulation and structural support for the CICC turns.
The CICC conduit is chosen to match the CTE (Coefficient of Thermal Expansion) of the superconducting material. The superconducting material is a brittle intermetallic that is formed by reaction at high temperatures. The CICC provides support to the brittle superconducting material and an enclosure for cooling fluid which is necessary for superconducting performance. Too much strain imparted to the superconducting material will also degrade performance. The CICC conduit is chosen to match the thermal expansion of the superconducting material from the reaction temperature to room temperature for coil fabrication and to cryogenic (e.g. 5K) temperature for superconductor operation.
To provide structural support and insulation the CICC conduit is surrounded by the insulating material. Stresses are induced into the structure (CICC coil with turns surrounded by insulating material, glass roving and epoxy) by reaction of Lorentz forces when the coil is energized and upon cooldown of the structure due to the difference in thermal expansion between the insulating material and the CICC conduit, the geometry of the coil, and the anisotropic nature of the thermal coefficient of expansion and of the anisotropic nature of the strength and modulus of elasticity due primarily to the 2D nature of the composite material of the insulation. By 2D nature, it is meant that in the direction perpendicular to the warp-fill plane, the composite exhibits epoxy-like properties. These resulting stresses in the insulation are very large and will likely crack in operation.
For some projects, the existing insulation design (using the 2D composite support and insulation system given above) results in unacceptably large stresses which violate the design guidelines and requirements. This provides risk of structural and electrical degradation or failure. Given the expense of the magnets and the associated projects, risk reduction and improved reliability achieved with this design appears prudent.